Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



Michael, You can do an overlay with the Sakrete Sand Mix Topping & Bedding Mix with the Sakrete Bonder & Fortifier. This material can be applied from 1/2" up to 2". All loose, brittle, soft and unsound material needs to be removed. Make sure that you clean the surface and remove any loose dust, dirt, debris, paints, and or sealers. Dampen the surface prior to application.
- Lee-Technical Service
Tuesday, May 24, 2016 at 10:24 AM

Justin, whether the concrete is poured at 2" or 8" you would need to incorporate a 1/2" expansion joint between the concrete and the brick. After the concrete has set up, you can come back and address the gap between the concrete and brick with a polyurethane caulk. For a 2" overlay you could also use the Bonder and Fortifier on the surface of the old concrete to help with the bond to the new concrete.
- Lee-Technical Service
Tuesday, May 24, 2016 at 10:18 AM

John, it will be hard to guess at what the issue may be without more information. If you could give us a call at 1-800-334-0784 so that we can further assist you.
- Lee-Technical Service
Tuesday, May 24, 2016 at 10:13 AM

I have a 3ft.x 3ft. Porch deck that is crumbling at the edges and corners. How do I repair this. I want to add at least the surface.
- michael from joppa
Monday, May 23, 2016 at 1:30 PM

i have had some work done around my house on the patio. when they replaced the patio they never poured the concrete to the old height now it looks like crap against the bricks on my house. what is the best way to pour concrete that is 2 inches thick and 8 inches high and get it to bond to the bricks?
- justin
Saturday, May 21, 2016 at 5:28 PM

My work area is 15" long by 12" wide by 2" deep. When I mixed the concrete, it was all gravel and hardly any concrete. So when I added it to the hole hoping for the best, I had what looked like a monument to a gravel driveway. No smoothness at all. Needless to say, the entire thing had to be dug out again. Now what?
- John Newell
Friday, May 20, 2016 at 8:00 PM

Gary, yes, you can add the Sakrete Bonder & Fortifier to the mix to give you a better adhesion to the stone. You should not have to brush a bonding agent over your scratch coat.
- Lee-Technical Service
Friday, May 20, 2016 at 1:07 PM

I put a mortar scratch coat on the existing brick fireplace. Now I want to cover it with a stone. Do I need to apply and adhesive or just use mortar. If I need to apply and adhesive can I just add something to the mortar mix rather than paint the adhesive on the scratch coat
- gary
Thursday, May 19, 2016 at 5:15 PM

Bob and Greg, you could use the Sakrete Sand Mix modified with the Sakrete Bonder & Fortifier to cap that slab. You will need to make sure that the slab has a rough surface and clean it to remove any loose dirt, dust, debris, paint, and or sealers. Frame around the slab to the 2” thickness that you need. Dampen the slab, mix and pour the material. Be sure to apply a broom finish to the surface before the material is completely set.
- Lee-Technical Service
Wednesday, May 11, 2016 at 4:32 PM

Alfred, it really depends on what the bed is made of. If you have pavers that are set on concrete then you can use the Sakrete Sand Mix Topping and Bedding Mix to grout in your pavers. If your pavers are on a sand bed and the joint width between them is no less than 1/4” and no greater than 1 1/2", then you could use Paverset Polymeric Sand or Permasand. If it is on a sand bed and your joints are less than 1/4” then there is nothing in our product line that can be used to bind them together.
- Lee-Technical Service
Wednesday, May 11, 2016 at 3:59 PM



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